Recently, nonvolatile memories capable of storing a large volume of data at a high speed have been developed along with the development of digital technologies.
A flash memory and a ferroelectric memory are well-known as such nonvolatile memories.
Among these nonvolatile memories, the flash memory includes a floating gate embedded in a gate insulating film of an insulated-gate field-effect transistor (IGFET), and stores information by accumulating electric charges indicating recording information, in the floating gate. However, there is a drawback that a relatively high voltage needs to be provided to such a flash memory since it is necessary to flow a tunnel current to the gate insulating film of the flash memory at the time of writing and erasing the information.
In contrast, the ferroelectric memory, which is also referred to as a ferroelectric random access memory (FeRAM), stores information by utilizing the hysteresis characteristic of a ferroelectric film formed in a ferroelectric capacitor. The ferroelectric film causes polarization in response to a voltage applied between upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is removed. When the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization is also reversed. By causing directions of the polarity to correspond respectively to “1” and “0,” the information is written in the ferroelectric film. Advantages of the FeRAM are that the voltage required for writing in the FeRAM is lower than that required for writing in the flash memory, and that it is possible to write information in the FeRAM at a higher speed than that of the flash memory. A system on chip (SOC), on which a FeRAM and a logic circuit are mixedly mounted, has been examined to be used for an IC card and the like by utilizing the above advantages.
A capacitor dielectric film provided to the ferroelectric capacitor is made of, for example, a PZT (Lead Zirconate Titanate: PbZrTiO3) film. There are various kinds of methods for forming the capacitor dielectric film.
For example, in Japanese Patent Application Laid-open Publication Hei 11-292626, the PZT film is formed by a sol-gel method using a solution in which an organometallic compound is dissolved in an organic solvent such as butanol. The sol-gel method has an advantage that costs for forming a film is lower than those of a sputtering method, a metal organic chemical vapor deposition (MOCVD) method or the like. Hence, the sol-gel method has been widely studied and developed.
In addition, when the ferroelectric capacitor is formed, thermal treatment is generally carried out in an oxygen atmosphere for the purpose of recovering damages and defects caused in the ferroelectric film. For this reason, an iridium oxide film is used as an upper electrode of the ferroelectric capacitor in some cases, because the iridium oxide film is not easily oxidized even in the oxygen atmosphere.
However, it is known that huge crystals made of abnormally-grown iridium oxide are easily generated on a surface of an iridium oxide film. The huge crystals deteriorate electric characteristics of the ferroelectric capacitor, and this may finally cause a decrease in yield of semiconductor devices.
To solve such a problem, in Japanese Patent Application Laid-open Publication No. 2001-127262, a two-step sputtering method is used to suppress generation of the aforementioned huge crystals. Two-step sputtering method includes the first step of forming a film by low sputtering power, and the second step of growing the film by high sputtering power, and these two steps are sequentially carried out (paragraph 0025).
In Japanese Patent Application Laid-open Publication No. 2000-91270 (JP No. 2000-91270 A), a laminated film which is configured by forming an iridium oxide film and an iridium film in this order, is used as an upper electrode. According to JP No. 2000-91270 A, the iridium oxide film of the lower layer prevents deterioration of capacitance characteristics, and the iridium film of the upper layer reduces resistance of the upper electrode (paragraph 0027).
In Japanese Patent Application Laid-open Publication No. 2002-246564, a PZT film formed by a sputtering method is crystallized by performing the first anneal on the PZT film (paragraph number 0035). Then, after an upper electrode made of iridium oxide is formed on the PZT film, the second anneal is carried out on the upper electrode (paragraph 0038).
In Japanese Patent Application Laid-open Publication No. 2005-183842, a laminated film formed of first and second conductive metal oxide films, both of which are made iridium oxide, is used as an upper electrode (paragraph 0035 to 0037).
Similarly, in Japanese Patent Application Laid-open Publication No. 2006-73648, a two-layered iridium oxide film is formed as an upper electrode (paragraph 0033).
On the other hand, in Japanese Patent Application Laid-open Publication No. 2001-237392, the PZT film is formed by a physical vapor deposition (PVD) method, a CVD method, the sol-gel method or the like, and an iridium film and an iridium oxide film are used as upper and lower electrodes (paragraph 0020 and 0021).
In Japanese Patent Application Laid-open Publication No. 2003-218325, a first PZT film in an amorphous state is formed by a sputtering method, and the PZT film is annealed to be crystallized. After that, a second PZT film is formed on the first PZT film by a MOCVD method (paragraph 0024 to 0027).
In Japanese Patent Application Laid-open Publication No. 2004-153006, a platinum oxide film is formed between an iridium oxide film constituting a lower electrode and a PZT film, in order to prevent iridium of the lower electrode from diffusing into the PZT film due to anneal at the time of crystallizing the PZT film (paragraph 0074).
In Japanese Patent Application Laid-open Publication No. 2004-296735, an oxygen-containing film and a barrier film are formed on a PZT film. Thereby, oxygen is supplied from the oxygen-containing film to the PZT film at the time of anneal the PZT film, and the barrier film prevents this oxygen from escaping upward (paragraph 0046).
Then, according to Japanese Patent Application Laid-open Publication No. 2004-214569, a decrease in switching charge of a capacitor is relieved by stacking a PZT film formed by the MOCVD method and a PZT film formed by the sputtering method in this order (paragraph 0049 and 0060).
On the other hand, in Japanese Patent Application Laid-open Publication No. Hei 9-260612, a capacitor dielectric film formed by stacking an SBT film, an SBTN film, and an SBT film in this order maintains residual spontaneous polarization and switching charge, and keeps coercive electric field and a leak current low (paragraph 0059).
In addition, in Japanese Unexamined Patent Application Publication No. Hei 5-347391 (JP No. Hei 5-347391 A), and Japanese Patent Application Laid-open Publications Nos. 2000-82792 and 2000-31403, a crystallized first ferroelectric film and an amorphous second ferroelectric film are formed in this order as a capacitor dielectric film (see, for example, paragraph 0007 in JP No. Hei 5-347391 A).